Lithographic masks have a pattern of features that are desired to be imparted into a workpiece, in order to fabricate, for example, semiconductor integrated circuits or other microminiature devices in the workpiece. Typically, the features of the mask are transferred to the workpiece by first placing the mask on the object plane of an optical imaging system that focuses optical radiation propagating through the mask, in order to transfer the features of the mask to a photoresist layer either overlying the workpiece or overlying a layer of material which overlies the workpiece. Then the photoresist layer is developed in order to form a patterned photoresist layer. Thus, the pattern of the mask features is transferred to the photoresist layer, with a magnification (greater to, equal to, or less than unity) that depends upon the design of the optical imaging system. The resulting pattern of the photoresist features is used to define features in the workpiece--such as edges of ion implant regions, edges of removed portions of the layer(s) of material overlying the workpiece to be removed, or edges of portions of the workpiece itself--all these edges thus being in accordance with the edges of the mask features.
A limiting factor in thus transferring the mask features to the photoresist layer is diffraction caused by the non-vanishing wavelength .lambda. (in the vacuum) of the optical radiation that is used in the optical imaging system. As a result of this diffraction, the edges of the images formed by the optical focusing system become less sharp than desired, whereby the resolution of the mask features when focused on the photoresist layer deteriorates. To improve this resolution, phase-shifting masks have been proposed. In these masks, the phases of the optical radiation propagating through the mask are shifted by, say, .phi. radian in some regions relative to others. For example, a typical portion 9 (FIG. 1) of a prior-art phase-shifting mask (hereinafter, "mask portion 9") has an opaque chrome (chromium) layer pair 13 located on a portion of the top substantially planar surface 12 of a raised (plateau) region 11 of a transparent, typically quartz, substrate 10. The region 11 has a width x and is thicker, by a distance h, than an adjacent "trench" region having a width w. The top surface 12 of the plateau region 11 which is substantially planar except for the presence of an indentation that forms an undesired defect region 21. As used herein, the term "substantially planar" refers to a situation in which any departures from planarity do not degrade, by more than desired amounts, the sharpness of images produced (e.g., in the photoresist layer) when the mask portion 9 is used in the optical imaging system.
The distance h corresponds to the optical phase shift .phi. (radian). Thus, (hn)/.lambda.-h.lambda.=.phi., or h=.phi..lambda./(n-1), where n is the refractive index of the quartz. Typically, .phi.=.pi. radian; so that typically, h=.pi..lambda./(n-1). When viewed from above, the contours of the chrome layer pair 13 depend upon the contours of the ultimately desired features in the workpiece (or in a layer overlying the workpiece).
One problem that can arise in using the phase-shifting mask portion 9 is the presence of the above-mentioned defect region 21. It should be understood that, in general, the height of the defect region 21 can vary from point to point (as indicated in FIG. 1); that is, the surface of the indentation region can have an irregular shape, such as was formed during earlier processing. Such a defect region can also be located in the plateau region 11. Also, there can be a plurality of such defect regions, each typically (but not necessarily) being located in a different plateau or trench region.
Obviously, when the mask portion 9 is used in the optical imaging system, the presence of the defect region 21 can deteriorate the image formed by the mask portion 9 on the photoresist layer located on the workpiece. Rather than discard entirely this mask 9 having such a defect region 21 (the mask being otherwise very valuable) it is desirable to have a method of repairing the mask, i.e., a method of removing the defect region 21 without damaging the rest of the phase-shifting mask 9 (not shown).
In the Fourth Hoya Photomask Symposium, held in Japan, a paper entitled "Submicron Photolithography using Phase-Shifting Mask" was delivered by N. Hasegawa et al. In that paper, a method was disclosed for repairing an indentation defect region in a phase-shifting mask. In accordance with at method, the defect region was present in a .pi.-radian optical phase-shifting layer ("main shifter" layer) underneath which there was located another .pi.-radian phase-shifting layer ("sub shifter" layer); and a barrier layer was located underneath the sub shifter layer--the barrier layer thus intervening between the bottom surface of the sub shifted layer and the top surface of a transparent substrate. This barrier layer served as an etch-stopping layer. To repair the mask, a region of both the main and the sub shifter layers laterally encompassing the defect region was etched down to the barrier layer--whereby the resulting phase shift in this (etched) region became equal to 2.pi., which of course is equivalent to a zero phase shift. In this way, when the mask is used in the optical imaging system, optical image deteriorations caused by the indentation defect region are ameliorated.
The need for the barrier layer in that method, however, entails a need for depositing on this barrier layer some sort of transparent material for the .pi. and/or 2.pi.-radian phase-shifting layers, such as chemically vapor deposited (CVD) silicon dioxide layers. The need for these deposited oxide layers complicates the fabrication of the mask's substrate, as well as the processing and etching of the chromium layer 13. This ned for deposited oxide layers also complicates any required subsequent cleaning of the surface of the mask portion 9--because a chromium layer is not as adherent to a deposited silicon dioxide layer as it is to a quartz substrate; and a deposited oxide layer is not as resistant as quartz to the cleaning procedures used for cleaning the mask. Moreover, the need for these deposited oxide layers has the disadvantage of requiring very close matching of the refractive indices of the various layers. Therefore, it would be desirable to have a method of repairing indentation defect regions without the need for an intervening etch-stopping layer.